a) Field of the invention
The present invention relates to an imaging apparatus having electrooptic devices preferable to endoscopes in which a stop diameter of an imaging lens is changed by interlocking with focusing adjustment of the imaging lens.
b) Description of the prior art
An imaging optical system for endoscopes in which a stop diameter of an imaging lens is changed by interlocking with focusing adjustment of the imaging lens has been described in the past in, for example, Japanese Patent Publication No. Sho 62-35090 and Japanese Patent Preliminary Publication No. Sho 63-78119. The optical system described in Sho 62-35090 is adapted to arrange slidably a holding frame for an objective lens provided in a non-flexible distal end portion of an endoscope in a longitudinal direction of the endoscope and interlock a member for limiting the aperture size of an aperture stop provided in the vicinity of the objective lens with the longitudinal movement of the holding frame so that when the holding frame is moved longitudinally by performing operation through an accessible control for focusing adjustment, the aperture size of the stop varies accordingly and as a result, brightness and the depth of field can be controlled automatically in accordance with the distance to an object. Further, the optical system stated in Sho 63-78119 is such that an electrochromic stop variable stepwise in aperture size is used as the aperture stop and the imaging optical system is constructed by combining the stop with a multi-focusing lens different in refracting power in accordance with the aperture size and is disposed in the distal end portion of the endoscope. In such an instance, when the aperture size of the electrochromic stop is changed, the position of a beam of light traversing the multi-focusing lens varies accordingly, with the result that the focal length of the imaging optical system depends on the aperture size and an in-focus position is shifted longitudinally. Thus, brightness and the depth of field can be controlled automatically in accordance with the distance to the object.
In the case of the optical system described in Sho 62-35090, however, it is substantially impossible, in view of a space, to accommodate the aperture stop variable mechanically in aperture size, the moving mechanism of the lens holding frame, and their interlocking mechanism in the non-flexible distal end portion of the endoscope of the distal end portion of a non-flexible endoscope. Further, in Sho 63-78119, problems have been encountered that it is considerably difficult that such a lens with a small diameter as used for the endoscope assumes multifocus and an extremely small electrochromic stop is hard to make.
Recently, an electrooptic stop apparatus such as a liquid crystal stop has been proposed and such a stop apparatus is advantageous to interlock the focusing adjustment of the imaging lens with the variation of the stop diameter of the imaging lens by the combination with the variable focusing lens such as a liquid crystal lens. A conventional example of the electrooptic stop apparatus is described in Japanese Patent Preliminary Publication No. Sho 59-156219. This conventional example is adapted to dispose closely a plurality of annular transparent electrodes concentric with an optical axis in a liquid crystal cell and supply selectively electric power to the electrodes, thereby changing a light transmitting area or light blocking area of the liquid crystal cell to be used as a variable stop.
Also, in the case of the conventional example in the foregoing, problems have arisen that since each electrode is provided with the connection of a lead wire for power supply, the electrode fails to take a complete annular form and clearance thus occurs between the electrodes adjacent to each other, with the result that light leaks out though the clearance even in a light blocking state. In addition, another problem has also arisen that the structure is complicated because the area occupied by the connection must be small to such extent as is possible.
In the case where the liquid crystal lens and the liquid crystal stop mentioned above are incorporated in, for example, an objective optical system of the endoscope, a miniaturized circle less than 10 mm in diameter is required and this has brought about difficulties that the connection protrudes from the peripheral edge of a liquid crystal optical element, with great inconvenience in practical use.